The present disclosure relates to optical devices for optical communications.
Optical isolator is an optical element that is often used to reduce the backward light in optical transmissions. Optical isolator is a nonreciprocal transmitting device. It allows a light from a light source to pass in a forward direction but prevents light to transmit in a backward direction, thus isolating the light source from the backward light. Free space isolator is a type of optical isolator often used in optical transceivers and tunable lasers.
A conventional free space isolator 10, referring to FIG. 1, can include a first polarizer 11, a Faraday rotator 12, and a second polarizer 13. The optical axes of the first polarizer 11 and the second polarizer 13 are oriented at a 45° angle. The Faraday rotator 12 is a nonreciprocal optical element. It can rotate the polarization of an incident light or a backward light by 45° along a same direction 15. After the incident light passes the first polarizer 11, its polarization is aligned to be parallel to the optical axis of the first polarizer 11. The polarization of the incident light is then rotated by 45° along the direction 15 by the Faraday rotator 12 such that the polarization becomes parallel to the optical axis of the second polarizer 13, allowing incident light to pass through the second polarizer 13.
Backward lights always exist in optical systems. Backward light can include unwanted or astray lights reflected or scattered from various optical elements in the optical system. A backward light typically has randomized polarizations. Its polarization is linearized to be parallel to the optical axis of the second polarizer 13 as it enters the free space isolator 10. The Faraday rotator then rotates the polarization of the backward light along the same direction 15, making its polarization perpendicular to the optical axis of the first polarizer 11. The backward light is thus blocked by the first polarizer 11, which isolates the backward light from the source direction of the incident light.
The above described conventional free space isolator has several drawbacks. It is rather expensive because of the costs of the two polarizing crystals (typically implemented by Polacors) and the Faraday rotator. There is thus a need for a simpler, effective, and less expensive optical isolator.